Boiling water nuclear reactors are known. Typically these reactors include a re-loadable cores including side-by-side fuel bundles. It is constructive in understanding the present invention to first summarize the construction of the fuel bundles, second to discuss how the fuel bundles operate within a reactor core, and finally to understand how reactors have in the past been reloaded.
Fuel bundles in a boiling water reactor contain sealed fuel rods having pellets of nuclear fuel in their interior. Groups of these fuel rods are combined to include a unitary assembly of upstanding fuel rods known as a fuel bundle.
Such fuel bundles include a lower tie plate supporting the group of fuel rods. This lower tie plate admits a flow of water coolant into the fuel bundle for the generation of steam. At least some of the fuel rods extend to and usually fasten to an upper tie plate, this upper tie plate permitting the exit of water coolant and generated steam. A fuel bundle channel extends around the tie plates and the fuel rods in between. This channel restricts the flow of coolant between the tie plates to isolate the steam and water flow path interior of a fuel bundle from all other fuel bundles and the immediately surrounding core bypass zone containing water moderator for neutron moderation. The fuel rods within the fuel bundle are flexible and if not constrained would move out of position and even come into abrading contact; consequently, so-called fuel bundle spacers are placed at spaced elevations in the fuel bundle to maintain the fuel rods in their designed side-by-side relation.
Fuel bundles are placed interior of a boiling water nuclear reactor in a cylindrical central grouping or reactor "core." This core contains many (300 to 1000) side-by-side standing fuel bundles. Water coolant is admitted to the fuel bundles through the lower tie plate from a lower plenum, heated to generate steam in passing through the fuel bundles as the fuel bundles undergo nuclear reaction, and discharged through the upper tie plate to an upper plenum where the generated steam is processed for the extraction of energy from the generated steam.
The reactivity of fuel bundles is typically controlled in so-called "cells", clusters of four fuel bundles, these elongated bundles being separated by a similarly elongated cruciform spatial interval. This elongated cruciform spatial interval is defined exterior of the channels of the fuel bundles. During operation of the reactor, this elongated cruciform section is flooded with water that assists in the moderation of the neutrons for continuing the nuclear reactor. When it is desired to either locally control or "shape" the nuclear reactor, or to shut down the nuclear reaction altogether, elongated cruciform control rods penetrate into and occupy the similarly elongated cruciform sectioned spatial intervals within each control cell. Control of the nuclear reaction results from this penetration.
The control rods when inserted absorb neutrons. For shaping of the reaction, the absorption locally moderates the nuclear reaction. For shutting down the reaction, the control rods absorb sufficient neutrons to cause the reactor to be subcritical; the neutrons emitted and moderated are insufficient to support a continuing reaction.
The refueling of such reactors can be summarized. Typically, the nuclear reactor undergo "outages", periods of time when the reactors are usually disassembled for replacement and relocation of fuel bundles within the reactor core. Typically, with each reactor outage, about 1/3 to 1/5 of the fuel bundles are replaced with new fuel bundles and the used expended fuel bundles discarded. Further, and usually as a function of in-service life, fuel bundles are relocated within the reactor on the basis of age.
A typical fuel bundle relocation occurs with respect to age or relative in-service life of the fuel bundles. Placement of new replacement fuel bundles has heretofore been restricted to a uniform distribution of a fuel bundle design or designs across the reactor core without regard to the design margins of the fuel bundles. Typically, the placement of the new and unused fuel bundles occurs to regions of the reactor core where the dynamics of the newly placed fuel essentially drives and shapes the desired overall power desired uniformly across the core in the fuel design; middle service life fuel bundles are evenly distributed throughout the core in balanced relationship to the newly added fuel; older and almost completely used fuel bundles are placed in a ring surrounding the periphery of the core for maintaining among other things neutron leakage from the core to a minimum. Thus one of the parameters or "degrees of freedom" that has been utilized in distributing fuel bundles in the past to a configuration within the reactor core has been the relative in-service life of the fuel bundles.
It has been known to accommodate in nuclear reactor cores at differing core locations, fuel bundles having contained fuel rods with different fuel pellet designs of differing reactivities. An example of a fuel bundle and fuel pellet design that incorporates differing reactivities is the addition of gadolinium to the fuel pellets within a fuel bundle for so-called "power shaping" of the fuel bundle during its operational life. Such power shaping can be best understood by first setting forth the general neutron density characteristics of a boiling water nuclear reactor and secondly setting forth how "power shaped" fuel bundles are distributed to specific locations within a boiling water nuclear reactor core.
The fuel bundles within a boiling water nuclear reactor include a lower "single phase" region and an upper "two phase" region. Simply stated, when water enters the lower part of a fuel bundle, no steam is present. As the water rises within the fuel bundle, increasing fractions of the upwardly rising fluid become steam. Hence, the upper two phase region of the fuel bundle is said to include an increasingly higher void fraction as the water proceeds from the bottom of the fuel bundle to the top of the fuel bundle.
This void fraction characteristic is not only common to the individual fuel bundles within a reactor core but common to the entire core as well. Simply stated, the bottom of the entire core has a relatively high density of water moderator; the top of the entire core has a relatively low density of water moderator (with a higher "void fraction" of steam).
These differing densities of water in the reactor core directly effect the nuclear "neutron" densities within a boiling water nuclear reactor core. The reader will remember that the main atomic reaction in a boiling water nuclear reactor generates high speed neutrons. These high speed neutrons must be moderated to low speed or "thermal" neutrons to continue the reaction. The water "moderator" within the reactor accomplishes this function.
Specifically, the water dense bottom of nuclear reactor cores tend to have areas of high thermal neutron densities extending outwardly to the periphery of the core. The top of nuclear reactor cores having a high water void fraction tend to have areas of lower neutron densities at the top of the core--especially in the peripheral regions of the core. Since the thermal neutron densities are directly related to the power outputs of nuclear fuel rods within the fuel bundles, it will be appreciated that this variability of neutron density has been directly related to the way in which a core is loaded with so-called "power shaped" fuel.
This described (and oversimplified) neutron density is complicated by another factor. This factor is the so-called "hot to cold reactivity swing." A brief summary of this problem will suffice for the purposes of this disclosure.
Reactors are most difficult to shut down from the nuclear reaction standpoint when the reactors are in the "cold" state and just starting up. In this cold state, the reactors have a high water content (there is little or no steam). Realizing this, it is a common regulatory requirement that the reactors be capable of being shut down in the cold state with that control rod having the highest efficiency or "worth" inoperative and not inserted to the core.
Understanding this much, it will be noted that hot reactivity is a more or less uniform phenomena throughout the reactor core. Cold reactivity, however, is a different matter. While cold reactivity in the general case is a uniform phenomena throughout the reactor, the withdrawal of the most critical control rod makes the cold reactivity measurement a matter of local concern.
Modern regulatory--and safe--nuclear practice requires that the ability to shut down the reactor in the difficult to shut down "cold" state be analyzed--at least on a theoretical basis. Further, and for the modeling of this capability, it is presumed that the particular control rod having the greatest moderating effect on the reaction is inoperative. With such a constraint, it must be shown analytically that any fuel loading design can be shut down in the cold state with a margin of at least one percent from the critical state.
This being the case, the reader will understand that cold reactivity is a decidedly local state. It depends not only on the core wide cold reactivity, but also on the local reactivity in the vicinity of that control rod having the most critical effect on shutting down the nuclear reaction in the cold state. It will further be understood, that these hot to cold reactivity swings constitute another of the complicated neutron density interactions necessary in modern boiling water nuclear reactor core design.
To accommodate these variations in neutron density, certain fuel bundle designs have been "power shaped" utilizing the burnable neutron absorber gadolinium within the fuel pellets. Specifically, and as to new fuel bundles placed within the outer 1/3 diameter of a reactor core, there is a tendency of the fuel rods contained in the lower region of the fuel bundle to have a higher linear heat generation rate than the same fuel rods in the upper two phase region of the fuel bundle. This difference in neutron density has been "shaped" by the placement of gadolinium typically in the lower regions of such fuel bundles. During the first (and part of the second) cycle of reactor life, the gadolinium absorbs the neutron surplus in the lower single phase region of such fuel bundles and prevents the linear heat generation rates of such fuel bundles from being exceeded.
It is to be understood that the absorption of neutrons in a boiling water nuclear reactor is never completely beneficial; such absorption of neutrons which do not contribute to the "chain reaction" always represents an inefficiency in the desired nuclear reaction. However, as to such fuel bundles in the early portion of their in-service lives, the benefits of preventing excessive linear heat generation rates far outweigh the detriment of neutron absorption. Unfortunately, and during the remainder of the fuel bundle life cycle, the remaining burned gadolinium neutron absorber continues to absorb neutrons at a reduced rate--essentially being completely parasitic and detrimental to the nuclear reaction.
There is an additional difficulty with such gadolinium "power shaped" designs. Specifically, the amount of the added gadolinium must be founded on a design prediction of the local power of the core as the core is configured after the reload. Thus a fuel designer first predicts this overall power and thereafter specifies the amount and location of gadolinium that must be added to the fuel bundle to achieve a theoretical efficiency. By way of example, if a 10% improvement of linear heat generation rate is desired, sufficient gadolinium is added to the fuel bundle to achieve this result.
Unfortunately, such design predictions are rarely exactly on target. Furthermore, and to complicate the process of design prediction, it will be remembered that the gadolinium --functioning as a neutron absorber--not only effects the neutron density locally within the bundle to which the gadolinium is added but also the locally surrounding fuel bundles--many of which are well into their in-service life. It will be appreciated that the process of predicting the performance of such fuel is complicated. By way of furthering the example give above, if a 10% improvement in linear heat generation rate was predicted and sufficient gadolinium added to achieve this result, the actual required gadolinium may have been different. For example, only an 8% margin may have been required; alternately, it may be found that a 12% margin was needed. In either case, the gadolinium design will be less than optimum representing a further inefficiency in the fuel cycle.